Methods for treating and/or limiting development of diabetes

ABSTRACT

The present invention provides methods for identifying candidate compounds for limiting development of and/or treating diabetes, and methods for limiting development of and/or treating diabetes.

CROSS-REFERENCE

This application claims priority to U.S. Provisional Patent Application Ser. No. 61/501,480 filed Jun. 27, 2011, incorporated by reference herein in its entirety.

INTRODUCTION

Voltage-gated calcium (Ca_(V)) channels are critical in β cell physiology and pathophysiology. They are not only take center stage in the regulation of insulin secretion, but are also involved in β cell development, survival and growth through the regulation of protein phosphorylation, gene expression and the cell cycle. The function and density of β cell Ca_(V) channels are regulated by a wide range of mechanisms either shared by other cell types or specific to β cells, e.g., channel phosphorylation, interaction with other molecules and glucose metabolism-derived signaling. Dysfunctional Ca_(V) channels causes β cell malfunction and even death as manifested in the most common metabolic disorder diabetes mellitus. Indeed, a T-lymphocyte-mediated autoimmune attack plays a crucial role in β cell death in type 1 diabetes. In addition, factors in type 1 diabetic serum compel unphysiological amounts of Ca²⁺ to enter pancreatic β cells through hyperactivation of β cell Ca_(V) channels resulting in β cell apoptosis. Undoubtedly, this process aggravates the disease development on top of the autoimmune attack. Such factors are also visualized in type 2 diabetic serum where they behave in the same way as they do in type 1 diabetic serum. In fact, reduction in β cell mass and hyperactivation of β cell Ca_(V) channels appear under type 2 diabetic conditions such as those in the Goto-Kakizaki rat.

It has been demonstrated that elevated apolipoprotein CIII (ApoCIII acts as a diabetogenic serum factor to drive β cell destruction via hyperactivation of β cell Ca_(V) channels. Moreover, we have recently shown that in vivo suppression of ApoCIII delays onset of diabetes in the BioBreeding™ rat. Normally, ApoCIII is a blood plasma component. It is synthesized predominantly in the liver and to a minor extent in the intestine. Liver and intestinal cells release this apolipoprotein into the blood where it is situated on the surface of chylomicrons, very low density lipoproteins (LDLs) and high density lipoproteins (IBMs). ApoCIII is composed of 79 amino acid residues that form six amphiphilic α-helixes, each containing about 10 residues. The three-dimensional NMR structure and dynamics of ApoCIII have been resolved when it complexes with sodium dodecyl sulfate micelles, mimicking its natural lipid-bound state. The six amphiphilic α-helixes assemble into a necklace-like chain wrapping around the sodium dodecyl sulfate micelle surface. Dogmatically, ApoCIII serves as an effective inhibitor of triglyceride hydrolysis by inhibiting lipoprotein lipase and through interference with triglyceride-rich lipoproteins binding to the negatively charged cell surface where lipoprotein lipases and lipoprotein receptors reside. It impedes the selective uptake of cholesteryl esters from LDL and HDL by binding to the scavenger receptor class B type I (SR-BI), and hampers the endocytosis of cholesterol-rich LDL by prevention of apolipoprotein B binding to LDL receptors. Elevated plasma ApoCIII concentration is a feature of dyslipidemia in obesity and observed in both type 1 and type 2 diabetes, whereas a group of Ashkennazi Jewish with reduced plasma ApoCIII concentration maintains cardiovascular health and greater insulin sensitivity with age and reaches exceptional longevity.

In addition to the dogmatic roles in lipid metabolism, ApoCIII is also a multifaceted player in cell signaling. It can bind to distinct cell surface receptors including scavenger receptor class B type I (SR-BI), Toll-like receptor 2 (TLR2) and uncharacterized binding sites relaying corresponding signals to their downstream effectors, e.g., β1 integrin, pertussis toxin-sensitive G proteins, NF-κB and protein kinases. However, nothing is known about molecular mechanisms whereby ApoCIII hyperactivates β cell Ca_(V) channels.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides methods identifying candidate compounds for limiting development of and/or treating diabetes, comprising

-   -   a) contacting a first population of insulin secreting cells with         an amount of apolipoprotein CIII (ApoCIII) effective to increase         density and/or conductivity of Ca_(v)1 channels, in the presence         of one or more test compounds; and     -   b) identifying those positive test compounds that inhibit an         ApoCIII-induced increase in density and/or conductivity of         Ca_(v)1 channels in the first population of insulin secreting         cells compared to control, wherein the positive test compounds         are candidate compounds for limiting development of and/or         treating diabetes.

In one embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of apolipoprotein Cud (ApoCIII) effective to increase density and/or conductivity of Ca_(v)1 channels, in the absence of one or more test compounds. This embodiment, which can be combined with the use of any of the other controls disclosed herein unless the context clearly dictates otherwise, may comprise, for example, contacting the control population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

In another embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds, and further contacting the control population of insulin secreting cells with Ca_(v)2 and/or Ca_(V)3 channel blockers, wherein positive test compounds that inhibit the ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the Ca_(v)2 and/or Ca_(V)3 channel blocker control population of insulin secreting cells to a greater degree than in the first population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes.

In another embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds, and further contacting the second population of insulin secreting cells with a Src kinase inhibitor and/or a protein kinase A (PKA) inhibitor, wherein positive test compounds that inhibit the ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the first population of insulin secreting cells to a greater degree than in the Src kinase inhibitor and/or a protein kinase A (PKA) inhibitor control population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes.

In a further embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds, and further contacting the control population of insulin secreting cells with a molecule that inhibits β1 integrin expression or activity, wherein positive test compounds that inhibit the ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the first population of insulin secreting cells to a greater degree than in the β1 integrin expression or activity control population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes.

As will be understood by those of skill in the art, a single control can be used in carrying out the methods of the invention, including but not limited to any of the controls disclosed above. Alternatively, multiple control embodiments can be used (2, 3, or more, including but not limited to any of the controls disclosed above), wherein each embodiment utilizes a different control cell population.

In a second aspect, the present invention provides methods for identifying candidate compounds for limiting development of and/or treating diabetes, comprising

-   -   (a) contacting a first population of insulin secreting cells         with an amount of ApoCIII effective to increase density and/or         conductivity of Ca_(v)1 channels, in the presence of one or more         test compounds; and     -   (b) identifying those positive test compounds that inhibit β1         integrin expression or activity in the first population of         insulin secreting cells compared to control, wherein the         positive test compounds are candidate compounds for limiting         development of and/or treating diabetes.

In one embodiment, the control comprises contacting a second population of insulin secreting cells contacted with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the absence of one or more test compounds. This embodiment may comprise, for example, contacting the second population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

In a third aspect, the present invention provides methods for identifying candidate compounds for limiting development of and/or treating diabetes, comprising

-   -   (a) contacting a first population of insulin secreting cells         with an amount of ApoCIII effective to increase density and/or         conductivity of Ca_(v)1 channels, in the presence of one or more         test compounds; and     -   (b) identifying those positive test compounds that inhibit         activation of PKA and/or Src kinase in the first population of         insulin secreting cells compared to control, wherein the         positive test compounds are candidate compounds for limiting         development of and/or treating diabetes.

In one embodiment, the control comprises contacting a second population of insulin secreting cells contacted with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the absence of one or more test compounds. This embodiment may comprise, for example, contacting the second population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

In various embodiments of any of these aspects of the invention, each of which can be combined except as clearly dictated otherwise by the context, the method comprises contacting the cells with ApoCIII for at least 6 hours; the candidate compounds are candidate compounds for limiting development of and/or treating type 1 diabetes; and/or wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 2 diabetes.

In a fourth aspect, the present invention provides methods for treating or limiting development of diabetes, comprising administering to a subject in need thereof with an amount effective of an inhibitor of PKA and Src kinase to treat or limit development of diabetes.

In a fifth aspect, the present invention provides methods for treating or limiting development of diabetes, comprising administering to a subject in need thereof with an amount effective of an inhibitor of β1 integrin expression and/or activity. In various embodiments, the inhibitor is selected from the group consisting of an anti-β1 integrin antibody, anti-β1 integrin aptamer, β1 integrin siRNA, β1 integrin shRNA, and β1 integrin antisense oligonucleotides.

In a sixth aspect, the present invention provides methods for treating or limiting development of diabetes, comprising administering to a subject in need thereof with an amount effective of an inhibitor of ApoCIII binding to pancreatic β cells. In various embodiments, the inhibitor is selected from the group consisting of aptamers and antibodies selective for ApoCIII.

DESCRIPTION OF THE FIGURES

FIG. 1. Apolipoprotein CIII incubation increases both the density and conductivity of Ca_(V)1 channels in β cells. (A) Examples of unitary Ca_(V)1 channel currents detected in plasma membrane patches of mouse islet 3 cells incubated with either vehicle solution as control or apolipoprotein CIII (ApoCIII). (B) Average number, open probability, mean closed time and mean open time of unitary Ca_(V)1 channels measured in plasma membrane patches attached to mouse islet β cells exposed to either control vehicle (n=33) or ApoCIII (n=32). (C) Examples of unitary Ca_(V)1 channel currents recorded in plasma membrane patches attached to either a control RINm5F cell or a cell treated with ApoCIII. (D) Average number, open probability, mean closed time and mean open time of unitary Ca_(V)1 channels detected in plasma membrane patches of control RINm5F cells (n=34) or cells incubated with ApoCIII (n=35). *P<0.05 and **P<0.01 versus control.

FIG. 2. Apolipoprotein CIII incubation increases whole-cell Ca²⁺ currents and coincubation with the Ca_(V)1 channel blocker nimodipine abrogates the effect of apolipoprotein CIII incubation in RINm5F cells. (A) Sample whole-cell Ca²⁺ current traces from a cell incubated with vehicle solution as control (cell capacitance: 10.1 pF) and apolipoprotein CIII (ApoCIII)-treated cell (cell capacitance: 11.1 pF). (B) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=26) and cells treated with ApoCIII (filled circles, n=26). *P<0.05 and **P<0.01 versus control. (C) Sample whole-cell Ca²⁺ current traces from a nimodipine (Nim)-incubated cell (cell capacitance: 10 pF) and a cell exposed to Nim together with ApoCIII (Nim/ApoCIII) (cell capacitance: 11.9 pF). (D) Average Ca²⁺ current density-voltage relationships in Nim-treated cells (open circles, n=20) and cells incubated with Nim/ApoCIII (filled circles, n=21). *P<0.05 and **P<0.01 versus Nim alone.

FIG. 3. PKA or Src kinase inhibition marginally reduces, but PKC inhibition does not affect apolipoprotein CIII-induced enhancement of whole-cell Ca²⁺ currents in RINm5F cells. (A) Sample whole-cell Ca²⁺ current traces from a cell incubated with vehicle solution as control (cell capacitance: 8.5 pF), an apolipoprotein CIII (ApoCIII)-treated cell (cell capacitance: 8.2 pF) and a cell exposed to ApoCIII plus the PKA inhibitor H-89 (ApoCIII/H-89, cell capacitance: 8.4 pF). (B) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=37) and cells treated with ApoCIII (filled circles, n=36) or ApoCIII/H-89 (filled triangles, n=36). *P<0.05 and **P<0.01 versus control. (C) Sample whole-cell Ca²⁺ current traces registered in a control cell (cell capacitance: 12.5 pF), an ApoCIII-incubated cell (cell capacitance: 12.0 pF) and a cell subjected to cotreatment with ApoCIII and the PKC inhibitor calphostin C (ApoCIII/CalpC, cell capacitance: 12.1 pF). (D) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=33), ApoCIII-treated cells (filled circles, n=33) and cells exposed to ApoCIII/CalpC (filled triangles, n=33). *P<0.05 and **P<0.01 ApoCIII versus control. ⁺P<0.05 and ⁺⁺P<0.01 ApoCIII/CalpC versus control. (E) Sample whole-cell Ca²⁺ current traces acquired in a control cell (cell capacitance: 9.5 pF), an ApoCIII-incubated cell (cell capacitance: 9.2 pF) and a cell exposed to ApoCIII together with the Src kinase inhibitor PP2 (ApoCIII/PP2, cell capacitance: 10.0 pF). (F) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=40) and cells incubated with ApoCIII (filled circles, n=40) or ApoCIII/PP2 (filled triangles, n=40). **P<0.01 ApoCIII versus control. ⁺P<0.05 ApoCIII/PP2 versus control.

FIG. 4. Combined inhibition of PKA, PKC and Src kinase counteracts apolipoprotein CIII-induced augmentation of whole-cell Ca²⁺ currents in RINm5F cells and coinhibition of PKA and Src kinase is sufficient to obtain this counteraction. (A) Sample whole-cell Ca²⁺ current traces registered in a vehicle-incubated cell (Control, cell capacitance: 7.9 pF), a cell subsequent to apolipoprotein (ApoCIII) treatment (cell capacitance: 7.0 pF) and a cell exposed to ApoCIII in the presence of the protein kinase inhibitor cocktail of H-89, calphostin C and PP2 (ApoCIII/H-89/CalpC/PP2, cell capacitance: 7.2 pF). (B) Average Ca²⁺ current density-voltage relationships in control cells (n=35) and cells exposed to ApoCIII (n=34) or to ApoCIII/H-89/CalpC/PP2=35). *P<0.05 versus control and apoCIII/H-89/CalpC/PP2. (C) Sample whole-cell Ca²⁺ current traces from a control cell (cell capacitance: 8.5 pF), a cell subsequent to ApoCIII treatment (cell capacitance: 8.2 pF) and a cell exposed to ApoCIII in the presence of the protein kinase inhibitors H-89 and PP2 (ApoCIII/H-89/PP2, cell capacitance: 8.7 pF). (D) Average Ca²⁺ current density-voltage relationships in control cells (n=26) and cells subjected to ApoCIII=26) or to ApoCIII/H-89/PP2 (n=27). *P<0.05 and **P<0.01 versus control; ⁺P<0.05 versus ApoCIII/H-89/PP2.

FIG. 5. Apolipoprotein CIII incubation does not alter β cell Ca_(V)1 channel expression. (A) Representative immunoblots of RINm5F cell homogenates, subjected to incubation with vehicle as control or apolipoprotein CIII (ApoCIII), probed with anti-Ca_(V)1.2, anti-Ca_(V)1.3 and anti-GAPDH antibodies, respectively. (B) Immunoblot quantification of the relative abundance of Ca_(V)1.2 (hatched column, n=6) and Ca_(V)1.3 subunits (filled column, n=6) in RINm5F cell homogenates subjected to ApoCIII incubation in comparison with control (open column, n=6). There was no significant difference in the relative abundance of total Ca_(V)1.2 and Ca_(V)1.3 subunits between control cells and cells incubated with ApoCIII (P>0.05).

FIG. 6. Knockdown of β1 integrin abrogates apolipoprotein CIII-induced enhancement of whole-cell Ca²⁺ currents in RINm5F cells. (A) Representative blots of β1 integrin- and GAPDH-immunoreactive bands in β1 integrin siRNA #1-, negative control siRNA (NC siRNA)- and β1 integrin siRNA #2-transfected cells. (B) Immunoblot quantifications of β1 integrin protein in NC siRNA—(open column, n=6), β1 integrin siRNA #1—(hatched column, n=6) and β1 integrin siRNA #2-transfected RINm5F cells (filled column, n=6). **P<0.01 versus NC siRNA. (C) Sample whole-cell Ca²⁺ current traces registered in individual cells following mock transfection and incubation with control vehicle (NO siRNA/Control, cell capacitance: 12.1 pF), NC siRNA transfection and control vehicle treatment (NC siRNA/Control, cell capacitance: 11.4 pF), NC siRNA transfection and apolipoprotein cm (ApoCIII) incubation (NC siRNA/ApoCIII, cell capacitance: 12.1 pF), β1 integrin siRNA transfection and exposure to vehicle solution (β1 integrin siRNA/Control, cell capacitance: 11.9 pF) and β1 integrin siRNA transfection and ApoCIII exposure (β1 integrin siRNA/ApoCIII, cell capacitance: 12.4 pF), respectively. (D) Ca²⁺ current density-voltage relationships in cells subjected to NO siRNA/Control (filled circles, n=29), NC siRNA/Control (open circles, n=28), NC siRNA/apoCIII (filled triangles, n=28), β1 integrin siRNA/Control (open triangles, n=29) and β1 integrin siRNA/ApoCIII (filled squares, n=29). *P<0.05 and **P<0.01 versus NO siRNA/Control, NC siRNA/Control and β1 integrin siRNA/Control. ⁺P<0.05 versus β1 integrin siRNA/ApoCIII.

FIG. 7. PKA, PKC or Src kinase inhibition does not alter whole-cell Ca²⁺ currents in RINm5F cells under basal conditions. (A) Sample whole-cell Ca²⁺ current traces from a vehicle-treated cell as control (cell capacitance: 8.8 pF) and a cell exposed to H-89 (cell capacitance: 8.5 pF). (B) Average Ca²⁺ current density-voltage relationships in control cells (open circles; n=20) and cells incubated with H-89 (filled circles, n=20). (C) Sample whole-cell Ca²⁺ current traces recorded in a control cell (cell capacitance: 10.4 pF) and a cell subjected to calphostin C incubation (CalpC, cell capacitance: 11.0 pF). (D) Average Ca²⁺ current density-voltage relationships in control cells (open circles; n=29) and cells exposed to CalpC (filled circles, n=29). (E) Sample whole-cell Ca²⁺ current traces obtained in a control cell (cell capacitance: 9.0 pF) and a PP2-treated cell (cell capacitance: 9.1 pF). (F) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=20) and cells incubated with PP2 (filled circles, n=19).

FIG. 8. Combined inhibition of PKA, PKC and Src kinase or coinhibition of PKA and Src kinase does not influence whole-cell Ca²⁺ currents in RINm5F cells under basal conditions. (A) Sample whole-cell Ca²⁺ current traces obtained in a cell incubated with vehicle solution as control (cell capacitance: 10.8 pF) and a cell heated with the protein kinase inhibitor cocktail composed of H-89, calphostin C and PP2 (H-89/CalpC/PP2, cell capacitance: 9.7 pF). (B) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=30) and cells treated with H-89/CalpC/PP2 (filled circles, n=30). (C) Sample whole-cell Ca²⁺ current traces obtained in a vehicle-treated cell as control (cell capacitance: 9.4 pF) and a cell treated with the protein kinase inhibitors H-89 and PP2 (H-89/PP2, cell capacitance: 9.1 pF). (D) Average Ca²⁺ current density-voltage relationships in control cells (open circles, n=24) and cells treated with H-89/PP2 (filled circles, n=24).

DETAILED DESCRIPTION OF THE INVENTION

All references cited are herein incorporated by reference in their entirety. Within this application, unless otherwise stated, the techniques utilized may be found in any of several well-known references such as: Molecular Cloning: A Laboratory Manual (Sambrook, et al., 1989, Cold Spring Harbor Laboratory Press), Gene Expression Technology (Methods in Enzymology, Vol. 185, edited by a Goeddel, 1991. Academic Press, San Diego, Calif.), “Guide to Protein Purification” in Methods in Enzymology (M. P. Deutshcer, ed., (1990) Academic Press, Inc.); PCR Protocols: A Guide to Methods and Applications (Innis, et al. 1990. Academic Press, San Diego, Calif.), Culture of Animal Cells: A Manual of Basic Technique, 2^(nd) Ed. (R. I. Freshney. 1987. Liss, Inc. New York, N.Y.), Gene Transfer and Expression Protocols, pp. 109-128, ed. E. J. Murray, The Humana Press Inc., Clifton, N.J.), and the Ambion 1998 Catalog (Ambion, Austin, Tex.).

As used herein, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. “And” as used herein is interchangeably used with “or” unless expressly stated otherwise.

All embodiments of any aspect of the invention can be used in combination, unless the context clearly dictates otherwise.

In a first aspect, the present invention provides methods identifying candidate compounds for limiting development of and/or treating diabetes, comprising

-   -   (a) contacting a first population of insulin secreting cells         with an amount of apolipoprotein CIII (ApoCH) effective to         increase density and/or conductivity of Ca_(v)1 channels, in the         presence of one or more test compounds; and     -   (b) identifying those positive test compounds that inhibit an         ApoCIII-induced increase in density and/or conductivity of         Ca_(v)1 channels in the first population of insulin secreting         cells compared to control, wherein the positive test compounds         are candidate compounds for limiting development of and/or         treating diabetes.

The inventors have discovered that ApoCIII incubation caused significant increases in Ca_(V)1 channel open probability and density at single channel levels. The treatment significantly enhanced whole-cell Ca²⁺ currents and the Ca_(V)1 channel blocker nimodipine completely abrogated the enhancement. The inventors have further discovered that coinhibition of PKA and Src kinase was sufficient for the same counteraction, and that knockdown of β1 integrin prevented ApoCIII from hyperactivating β cell Ca_(V) channels. Thus, the inventors have pinpointed therapeutic targets for limiting Ca⁺-dependent pancreatic β cell death, and thus for limiting and/or treating diabetes mellitus. Thus, the methods of this aspect of the invention can be used to identify compounds for limiting Ca²⁺-dependent pancreatic β cell death and thus for limiting development of and/or treating diabetes.

As used herein, “apoCIII” refers to a protein comprising the amino acid sequence shown in SEQ ID NO:2 (Human) (NCBI accession number CAA25233), SEQ ID NO:4 (Rat) (NCBI accession number AA40746), or SEQ ID NO:6 (Macaque) (NCBI accession number CAA48419), or functional equivalents thereof.

The apoCIII may be substantially purified apoCIII, available, for example, from Sigma Chemical Company (St. Louis, Mo.), wherein “substantially purified” means that it is removed from its normal in vivo cellular environment. Alternatively, the apoCIII may be present in a mixture, such as blood serum from type 1 diabetic or partially or fully purified therefrom using standard techniques, such as those described below. In a preferred embodiment, substantially purified apoCIII is used.

As discussed below, there are three known isoforms of human apoCIII that have the same amino acid sequence, but which differ in their glycosylation pattern. Thus, in a preferred embodiment, glycosylated apoCIII is used, wherein the glycosylation is preferably sialylation. In another preferred embodiment, mono-sialylated or di-sialylated apoCIII is used. Such glycosylated forms may be purchased, for example, from Sigma Chemical Company, or may be partially or fully purified using standard techniques, such as those described below.

Any suitable insulin secreting cell can be used, including but not limited to pancreatic β cells. As used herein, “pancreatic β cells” are any population of cells that contain pancreatic β islet cells. The cells can be obtained from any mammalian species, or may be present within the mammalian species when the assays are conducted in vivo. Such pancreatic β islet cell populations include the pancreas, isolated pancreatic islets of Langerhans (“pancreatic islets”), isolated pancreatic β islet cells, and insulin secreting cell lines. Methods for pancreatic isolation are well known in the art, and methods for isolating pancreatic islets, can be found, for example, in Cejvan et al., Diabetes 52:1176-1181 (2003); Zambre et al., Biochem. Pharmacol. 57:1159-1164 (1999), and Fagan et al., Surgery 124:254-259 (1998), and references cited therein. Insulin secreting cell lines are available from the American Tissue Culture Collection (“ATCC”) (Rockville, Md.). In a further embodiment where pancreatic β cells are used, they are obtained from ob/ob mice, which contain more than 95% β cells in their islets, and are commercially available.

Measuring the density and/or conductivity of Ca_(v)1 channels can be carried out by standard methods in the art, including but not limited to single channel and whole-cell patch-clamp measurements (cell-attached and perforated whole-cell patch-clamp techniques). As used herein, “increase density and/or conductivity of Ca_(v)1 channels” refers to increasing during the course of the assay above that seen in the absence of test compounds. The method does not require a specific amount of increase in density and/or conductivity of Ca_(v)1 channels over baseline, so long as the compound(s) promotes an increase in density and/or conductivity of Ca_(v)1 channels above that seen in the absence of test compounds. In a preferred embodiment, the increase is a statistically significant increase as judged by standard statistical analysis.

The contacting of the pancreatic β cells with the apoCIII may occur before, after, or simultaneously with contacting the cells with one or more test compounds. The contacting can be in vitro or in vivo (ex: in an experimental animal model). Any suitable culture conditions can be used for carrying out the methods of any of the candidate identification methods of the invention. In one embodiment, the cells are contacted with ApoCIII for at least 6 hours. In another embodiment, the cells are grown in medium comprising between 1 mM and 15 mM glucose; preferably between 3 in M and 12 mM; preferably about 11 mM glucose. In a further embodiment, the cells are cultured at approximately 37° C. (preferably in a humidified incubator, such as 5% CO₂) prior to recording the density and/or conductivity of the Ca_(v)1 channels at approximately room temperature. These and other suitable assay conditions are well within the level of those of skill in the art, based on the teachings herein.

In one embodiment, the candidate compounds are candidate compounds for limiting development of and/or treating type 1 diabetes. In another embodiment, the candidate compounds are candidate compounds for limiting development of and/or treating type 2 diabetes. The present invention further provides compounds identified by the above screening methods, and their use for treating subjects in need thereof.

In another embodiment, the methods further comprise large-scale synthesis of the test compounds that inhibit apoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the pancreatic β cells.

When the test compounds comprise polypeptide sequences, such polypeptides may be chemically synthesized or recombinantly expressed. Recombinant expression can be accomplished using standard methods in the art, as disclosed above. Such expression vectors can comprise bacterial or viral expression vectors, and such host cells can be prokaryotic or eukaryotic. Synthetic polypeptides, prepared using the well-known techniques of solid phase, liquid phase, or peptide condensation techniques, or any combination thereof, can include natural and unnatural amino acids. Amino acids used for peptide synthesis may be standard Boc (Nα-amino protected Nα-t-butyloxycarbonyl)amino acid resin with standard deprotecting, neutralization, coupling and wash protocols, or standard base-labile Nα-amino protected 9-fluorenylmethoxycarbonyl (Fmoc) amino acids. Both Fmoc and Boc Nα-amino protected amino acids can be obtained from Sigma, Cambridge Research Biochemical, or other chemical companies familiar to those skilled in the art. In addition, the polypeptides can be synthesized with other Nα-protecting groups that are familiar to those skilled in this art. Solid phase peptide synthesis may be accomplished by techniques familiar to those in the art and provided, such as by using automated synthesizers.

When the test compounds comprise antibodies, such antibodies can be polyclonal or monoclonal. The antibodies can be humanized, fully human, or murine forms of the antibodies. Such antibodies can be made by well-known methods, such as described in Harlow and Lane, Antibodies; A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., (1988).

When the test compounds comprise nucleic acid sequences, such nucleic acids may be chemically synthesized or recombinantly expressed as well. Recombinant expression techniques are well known to those in the art (See, for example, Sambrook, et al., 1989, supra). The nucleic acids may be DNA or RNA, and may be single stranded or double. Similarly, such nucleic acids can be chemically or enzymatically synthesized by manual or automated reactions, using standard techniques in the art. If synthesized chemically or by in vitro enzymatic synthesis, the nucleic acid may be purified prior to introduction into the cell. For example, the nucleic acids can be purified from a mixture by extraction with a solvent or resin, precipitation, electrophoresis, chromatography, or a combination thereof. Alternatively, the nucleic acids may be used with no or a minimum of purification to avoid losses due to sample processing.

When the test compounds comprise compounds other than polypeptides, antibodies, or nucleic acids, such compounds can be made by any of the variety of methods in the art for conducting organic chemical synthesis.

In one embodiment, the control comprises contacting a second population of insulin secreting cells with an amount of apolipoprotein (ApoCIII) effective to increase density and/or conductivity of Ca_(v)1 channels, in the absence of one or more test compounds. This embodiment, which can be combined with the use of any of the other controls disclosed herein, may comprise, for example, contacting the second population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

In one embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds, and further contacting the control population of insulin secreting cells with Ca_(v)2 and/or Ca_(v)3 channel blockers, including but not limited to ω-agatoxin IVA, α-conotoxin GVIA and SNX 482 (Ca_(v)2 channel blockers); and mibefradil and NNC 55-0396 (Ca_(V)3 channel blockers), wherein positive test compounds that inhibit the Apoeffi-induced increase in density and/or conductivity of Ca_(v)1 channels in the Ca_(v)2 and/or Ca_(V)3 channel blocker-control population of insulin secreting cells to a greater degree than in the first population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes. In this embodiment, the Ca_(v)2 and/or Ca_(V)3 channel blocker are selective for the Ca_(v)2 and/or Ca_(V)3 channel, and do not serve as a Ca_(v)1 channel blocker. It is within the level of those of skill in the art to determine, based on the teachings herein, the amount of any Ca_(v)2 and/or Ca_(V)3 channel blocker(s) that can be usefully used in a given assay.

In another embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds, and further contacting the control population of insulin secreting cells with a Src kinase inhibitor and/or a PKA inhibitor, wherein positive test compounds that inhibit the ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the first population of insulin secreting cells to a greater degree than in the Src kinase inhibitor and/or a PKA inhibitor control population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes. Exemplary. Src kinase inhibitors include PP1 analogs, PP2, and compounds disclosed in the examples that follow. Exemplary PKA inhibitors include adenosine 3′,5′-cyclic monophosphorothioate-R, H-7, H-8, H-9, H-89, and compounds disclosed in the examples that follow.

As shown in the examples that follow, the inventors have discovered that Apo=hyperactivates β cell Ca_(V)1 channels through integrin-dependent co-activation of PKA and Src kinase. Thus, inhibitors of PKA and/or Src should down-regulate positive candidate compounds of the present invention. Any suitable PKA and/or Src kinase inhibitor can be used, including but not limited to those disclosed in the examples that follow. It is within the level of those of skill in the art to determine, based on the teachings herein, the amount of any Src kinase inhibitor(s) and/or a PKA inhibitor(s) that can be usefully used in a given assay.

In a further embodiment, the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds and further contacting the control population of insulin secreting cells with a molecule that inhibits β1 integrin expression or activity, wherein positive test compounds that inhibit the ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the first population of insulin secreting cells to a greater degree than in the β1 integrin inhibitor control population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes. As shown in the examples that follow, the inventors have discovered that ApoCIII hyperactivates β cell Ca_(V)1 channels through β1 integrin-dependent coactivation of PKA and Src kinase. Thus, inhibitors of β1 integrin should down-regulate positive candidate compounds of the present invention. Any suitable β1 integrin inhibitor can be used (antibodies, antisense, siRNA, shRNA, etc.), including but not limited to those disclosed in the examples that follow. It is within the level of those of skill in the art to determine, based on the teachings herein, the amount of any β1 integrin inhibitor(s) that can be usefully used in a given assay.

As will be understood by those of skill in the art, a single control can be used in carrying out the methods of the invention, including but not limited to any of the controls disclosed above. Alternatively, multiple control embodiments can be used (2, 3, or more, including but not limited to any of the controls disclosed above), wherein each embodiment utilizes a different control cell population.

In a second aspect, the present invention provides methods for identifying candidate compounds for limiting development of and/or treating diabetes, comprising

-   -   (a) contacting a first population of insulin secreting cells         with an amount of ApoCIII effective to increase density and/or         conductivity of Ca_(v)1 channels, in the presence of one or more         test compounds; and     -   (b) identifying those positive test compounds that inhibit β1         integrin expression or activity in the first population of         insulin secreting cells compared to control, wherein the         positive test compounds are candidate compounds for limiting         development of and/or treating diabetes.

In one embodiment, the control comprises contacting a second population of insulin secreting cells contacted with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the absence of one or more test compounds. This embodiment may comprise, for example, contacting the second population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

All embodiments of the first aspect of the invention can be used in this second aspect unless the context clearly dictates otherwise.

In one embodiment, the control comprises contacting a second population of insulin secreting cells contacted with ApoCIII in the absence of test compounds. This embodiment may comprise, for example, contacting the second population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

In a third aspect, the present invention provides methods for identifying candidate compounds for limiting development of and/or treating diabetes, comprising

-   -   (a) contacting a first population of insulin secreting cells         with an amount of ApoCIII effective to increase density and/or         conductivity of Ca_(v)1 channels, in the presence of one or more         test compounds; and     -   (b) identifying those positive test compounds that inhibit         activation of PKA and/or Src kinase in the first population of         insulin secreting cells compared to control,

wherein the positive test compounds are candidate compounds for limiting development of and/or treating diabetes.

In one embodiment, the methods comprise identifying inhibitors of β1 integrin-mediated activation of PKA and/or Src kinase.

In one embodiment, the control comprises contacting a second population of insulin secreting cells contacted with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the absence of one or more test compounds. This embodiment may comprise, for example, contacting the second population of cells with a formulation, such as a buffer, that is similar to or identical to the formulation the test compounds are dissolved in.

All embodiments of the first aspect of the invention can be used in this third aspect unless the context clearly dictates otherwise.

In a further aspect, the present invention provides methods for treating or limiting development of diabetes, comprising administering to a subject in need thereof an amount effective of an inhibitor of PKA and Src kinase to treat or limit development of diabetes. Exemplary Src kinase inhibitors include PP1 analogs, PP2, and compounds disclosed in the examples that follow. Exemplary PKA inhibitors include adenosine 3′,5′-cyclic monophosphorothioate-R H-7, H-8, H-9, H-89, and compounds disclosed in the examples that follow.

In another aspect, the present invention provides methods for treating or limiting development of diabetes, comprising administering to a subject in need thereof with an amount effective of an inhibitor of β1 integrin expression and/or activity. In various embodiments, the inhibitor is selected from the group consisting of an anti-β1 integrin antibody, anti-β1 integrin aptamer, β1 integrin siRNA, β1 integrin shRNA, and β1 integrin antisense oligonucleotides.

In a still further aspect, the present invention provides methods for treating or limiting development of diabetes, comprising administering to a subject in need thereof with an amount effective of an inhibitor of ApoCIII activation of pancreatic β cells.

As used herein, an “inhibitor” of apoCIII activation includes compounds that reduce the transcription of apoCIII DNA into RNA, compounds that reduce translation of the apoCIII RNA into protein, and compounds that reduce the function of apoCIII protein. Such inhibiting can be complete inhibition or partial inhibition, such that the expression and/or activity of the apoCIII is reduced, resulting in a reduced ability to increase intracellular calcium concentration. Such inhibitors are selected from the group consisting of antibodies that bind to apoCIII; aptamers that can interfere with apoCIII activity; antisense oligonucleotides directed against the apoCIII protein, DNA, or mRNA; small interfering RNAs (siRNAs) or short hairpin RNAs (shRNAs) directed against the apoCIII protein, DNA, or mRNA, and any other chemical or biological compound that can interfere with apoCIII activity.

In one embodiment of each of these therapeutic aspects, the method is for treating diabetes. In this embodiment, the subject has been diagnosed with type 1 or type 2 diabetes. As used herein, “diabetes” is characterized by insufficient or no production of insulin by the pancreas, leading to high blood sugar levels.

As used herein, “treating diabetes” means accomplishing one or more of the following: (a) reducing the severity of the diabetes or diabetic complications; (b) limiting or preventing development of diabetic complications; (c) inhibiting worsening of diabetic complications or of symptoms characteristic of diabetes; (d) limiting or preventing recurrence diabetic complications or of symptoms characteristic, of diabetes; (e) limiting or preventing recurrence of diabetic complications or of symptoms characteristic of diabetes in patients that were previously symptomatic.

Symptoms characteristic of diabetes include, but are not limited to, elevated blood glucose levels, decreased insulin production, insulin resistance, proteinuria, and impaired glomerular clearance. Diabetic complications that can be treated according to the methods of the invention include, but are not limited to, complications in the nerves (such as diabetic neuropathy) and complications associated with smooth muscle cell dysregulaton (including but not limited to erectile dysfunction, bladder dysfunction, and vascular complications including but not limited to atherosclerosis, stroke, and peripheral vascular disease)

In another embodiment, the method is for limiting development of diabetes. In this aspect, the subject is at risk of type 1 or type 2 diabetes, and a benefit is to limit development of diabetes and/or diabetic complications. Any subject at risk of developing diabetes can be treated, including but not limited to subjects with one or more of, metabolic syndrome, known genetic risk factors for diabetes, a family history of diabetes, and obesity.

In a further embodiment, the methods for treating or limiting development of diabetes and/or diabetic complications further comprises treating those individuals that have been identified as overexpressing apoCIII compared to control. Increases in apoCIII expression precede development of diabetic complications, and thus this embodiment permits early detection of suitable patients for treatment using the methods of the invention.

As used herein, “overexpression” is any amount of apoCIII expression above control. Any suitable control can be used, including apoCIII expression levels from a subject known not to be suffering from diabetes, or previously determined standardized expression levels of apoCIII from a population of similar patient samples. Any amount of increased apoCIII expression relative to control is considered “overexpression”; in various embodiments, the overexpression comprises at least 10%, 20%; 50%, 100%, 200%, or greater increased apoCIII expression compared to control. In a preferred embodiment, apoCIII expression is detected in blood or serum samples. In one embodiment to evaluate the levels of apoCIII in pos, neg, and control sera, albumin is removed from serum samples using standard techniques, such as via use of Montage Albumin Deplete Kit (Millipore) or AlbuSorb™ (Biotech Support Group). The collected sera samples can then be freeze-dried overnight and run on sep-Pak C18. The eluted proteins can be freeze-dried and thereafter dissolved in 100 μL 0.1% TFA and run on an ACE C18 10-×0.21-cm column 20-60%, and the area under the curve, where apoCIII elutes, evaluated. ApoCIII can be identified using any suitable technique, including but not limited to MALDI mass spectrometry.

As used herein, the term “subject” or “patient” is meant any subject for which therapy is desired, including humans, cattle, dogs, cats, guinea pigs, rabbits, rats, mice, insects, horses, chickens, and so on. Most preferably, the subject is human.

The therapeutic may be administered by any suitable route, including but not limited to oral, topical, parenteral, intranasal, pulmonary, or rectal in dosage unit formulations containing conventional non-toxic pharmaceutically acceptable carriers, adjuvants and vehicles. The term parenteral as used herein includes percutaneous, subcutaneous, intravascular (e.g., intravenous), intramuscular, or intrathecal injection or infusion techniques and the like. In addition, there is provided a pharmaceutical formulation comprising a compound of the invention and a pharmaceutically acceptable carrier The therapeutic may be present in association with one or more non-toxic pharmaceutically acceptable carriers and/or diluents and/or adjuvants, and if desired other active ingredients. The therapeutic may be in a form suitable for oral use, for example, as tablets, troches, lozenges, aqueous or oily suspensions, dispersible powders or granules, emulsion, hard or soft capsules, or syrups or elixirs.

The dosage range depends on the choice of the compound, the route of administration, the nature of the formulation, the nature of the subject's condition, and the judgment of the attending practitioner. For example, oral administration would be expected to require higher dosages than administration by intravenous injection. Variations in these dosage levels can be adjusted using standard empirical routines for optimization, as is well understood in the art.

EXAMPLE 1 Apolipoprotein CIII Hyperactivates β Cell Ca_(V)1 Channels Through β1 Integrin-Dependent Coactivation of PKA and Src Kinase

Summary

Apolipoprotein CIII (ApoCIII) not only serves as an inhibitor of triglyceride hydrolysis, but also participates in diabetes-related pathological events such as the inflammatory process and hyperactivation of voltage-gated Ca²⁺ (Ca_(V)) channels in the pancreatic β cell. However, nothing is known about the molecular mechanisms whereby ApoCIII hyperactivates β cell Ca_(V) channels. We now demonstrate that ApoCIII increased Ca_(V)1 channel open probability and density. ApoCIII enhanced whole-cell Ca²⁺ currents and the Ca_(V)1 channel blocker nimodipine completely abrogated this enhancement. The effect of ApoCIII was not significantly influenced by individual inhibition of PKA, PKC or Src kinase. However, combined inhibition of PKA, PKC and Src kinase counteracted the effect of ApoCIII, similar results obtained by coinhibition of PKA and Src kinase. Moreover, knockdown of β1 integrin prevented ApoCIII from hyperactivating β cell Ca_(V) channels. These data reveal that ApoCIII hyperactivates β cell Ca_(V)1 channels through β1 integrin-dependent coactivation of PKA and Src kinase.

Introduction

Voltage-gated calcium (Ca_(V)) channels are critical in pancreatic β cell physiology and pathophysiology (Yang and Berggren, 2005; Yang and Berggren, 2006). They not only take center stage in the regulation of insulin secretion, but are also involved in β cell development, survival and growth through the regulation of protein phosphorylation, gene expression and the cell cycle (Yang and Berggren, 2005; Yang and Berggren, 2006). The function and density of β cell Ca_(V) channels are regulated by a wide range of mechanisms either shared by other cell types or specific to β cells, e.g., channel phosphorylation, interaction with other molecules and glucose metabolism-derived signaling (Catterall, 2000; Yang and Berggren, 2005; Yang and Berggren, 2006). Dysfunctional Ca_(V) channels cause β cell malfunction and even death as manifested in the most common metabolic disorder diabetes mellitus (Yang and Berggren, 2005; Yang and Berggren, 2006). Indeed, a T-lymphocyte-mediated autoimmune attack plays a crucial role in β cell death in type 1 diabetes. In addition, factors in type 1 diabetic serum compel unphysiological amounts of Ca²⁺ to enter pancreatic β cells through hyperactivation of β cell Ca_(V) channels resulting in β cell apoptosis. Undoubtedly, this process aggravates the disease development on top of the autoimmune attack (Yang and Berggren, 2005; Yang and Berggren, 2006). Such factors are also visualized in type 2 diabetic serum where they behave in the same way as they do in type 1 diabetic serum (Juntti-Berggren et al., 1993; Juntti-Berggren et al., 2004; Sol et al., 2009). In fact, reduction in β cell mass and hyperactivation of β cell Ca_(V) channels appear under the type 2 diabetic conditions such as those in the Goto-Kakizaki rat (Kato et al., 1996).

It has been demonstrated that elevated apolipoprotein CIII (ApoCIII) acts as a diabetologenic serum factor to drive β cell destruction via hyperactivation of β cell Ca_(V) channels (Juntti-Berggren et al., 2004; Sol et al., 2009). Moreover, we have recently shown that in vivo suppression of ApoCIII delays onset of diabetes in the BioBreeding rat, a rat model for human type 1 diabetes (Holmberg et al., 2011). Normally, ApoCIII is a blood plasma component. It is synthesized predominantly in the liver and to a minor extent in the intestine. Liver and intestinal cells release this apolipoprotein into the blood where it is situated on the surface of chylomicrons, very low density lipoproteins (LDLs) and high density lipoproteins (HDLs) (Gangabadage et al., 2008; Jong et al., 1999). ApoCIII is composed of 79 amino acid residues that form six amphiphilic α-helixes, each containing about 10 residues. The three-dimensional NMR structure and dynamics of ApoCIII have been resolved when it complexes with sodium dodecyl sulfate micelles, mimicking its natural lipid-bound state. The six amphiphilic α-helixes assemble into a necklace-like chain wrapping around the sodium dodecyl sulfate micelle surface (Gangabadage et al., 2008). Dogmatically, ApoCIII serves as an effective inhibitor of triglyceride hydrolysis by inhibiting lipoprotein lipase and through interference with triglyceride-rich lipoproteins binding to the negatively charged cell surface where lipoprotein lipases and lipoprotein receptors reside (Gangabadage et al., 2008; Jong et al., 1999). It impedes the selective uptake of cholesteryl esters from LDL and HDL by binding to the scavenger receptor class B type I (SR-BI), and hampers the endocytosis of cholesterol-rich LDL by prevention of apolipoprotein B binding to LDL receptors (Clayey et al., 1995; Huard et al., 2005; Xu et al., 1997). Elevated plasma ApoCIII concentration is a feature of dyslipidemia in obesity and observed in both type 1 and type 2 diabetes (Chan et al., 2002; Juntti-Berggren et al., 2004; Simdsten et al., 2008), whereas a group of Ashkennazi Jewish with reduced plasma ApoCIII concentration maintains cardiovascular health and greater insulin sensitivity with age and reaches exceptional longevity (Atanon et al., 2006).

In addition to the dogmatic roles in lipid metabolism, ApoCIII is also a multifaceted player in cell signaling. It can bind to distinct cell surface receptors including scavenger receptor class B type I (SR-BI), Toll-like receptor 2 (TLR2) and uncharacterized binding sites relaying corresponding signals to their downstream effectors, e.g., β1 integrin, pertussis toxin-sensitive G proteins, NF-κB and protein kinases (Fang and Liu, 2000; Kawakami et al., 2006; Kawakami et al., 2007; Xu et al., 1997). However, nothing is known about the molecular mechanisms whereby ApoCIII hyperactivates β cell Ca_(V) channels. In the present study, we demonstrate that ApoCIII upregulates β cell Ca_(V)1 channels through β1 integrin-dependent coactivation of PKA and Src kinase.

Results

Apolipoprotein CIII Increases Ca_(v)1 Channel Density and Conductivity in the β Cell

Our previous work reveals that ApoCIII incubation significantly enhances whole-cell Ca²⁺ currents in the mouse islet β cell (Juntti-Berggren et al., 2004). To clarify what type of β cell Ca_(V) channels and whether the density or conductivity was affected, we analyzed unitary Ca_(V)1 channel currents, characterized by a large unitary Ba²⁺ conductance with long-lasting openings, in mouse islet β cells (FIG. 1A) and RINm5F cells (FIG. 1C) following ApoCIII incubation. In experiments with mouse islet β cells, we observed more Ca_(V)1 channels, reflected by more layers of unitary Ba²⁺ currents, in plasma membrane patches of ApoCIII-treated cells than in those of control cells (FIG. 1A). The average number, open probability and mean open time of unitary Ca_(V)1 channels in ApoCIII-treated cells (n=32) were significantly greater than those in cells exposed to control vehicle (n=33) (FIG. 1B). The mean closed time of unitary Ca_(V)1 channels recorded in patches of ApoCIII-incubated cells was significantly shorter than that in control patches (FIG. 1B). Likewise, similar effects of ApoCIII occurred on Ca_(V)1 channels in insulin-secreting RINm5F cells. Plasma membrane patches of ApoCIII-incubated cells accommodated more Ca_(V)1 channels in comparison with those of vehicle-treated cells (FIG. 1C). Ca_(V)1 channels in the former opened more frequently than those in the latter (FIG. 1C). ApoCIII incubation (n=35) significantly increased channel number, elevated open probability, prolonged mean open time and shortened mean closed time of Ca_(V)1 channels as compared with incubation with vehicle solution (n=34) (FIG. 1D). Obviously, the data reveal that ApoCIII increased both density and conductivity of β cell Ca_(V)1 channels.

Pharmacological Ablation of Ca_(V)1 Channels Prevents Apolipoprotein CIII-Induced Hyperactivation of β Cell Ca_(V) Channels

The verification of the effects of ApoCIII on Ca_(V)1 channels by single channel analysis does not necessarily mean that ApoCIII only attacks Ca_(V)1 channels. To examine if the effects also occur on Other types of Ca_(V) channels, we analyzed whole-cell Ca²⁺ currents in RINm5F cells following ApoCIII incubation in the absence and presence of the Ca_(V)1 channel blocker nimodipine. Whole-cell Ca²⁺ currents in cells incubated with ApoCIII were larger than those in cells treated with vehicle solution (FIG. 2A). Whole-cell Ca²⁺ current densities observed in the voltage range from 10 to 30 mV in the ApoCIII group were significantly higher than those in the control group (FIG. 2B). In striking contrast, whole-cell Ca²⁺ currents were similar between control cells and cells incubated with ApoCIII in the presence of nimodipine (FIG. 2C). There was no significant difference in the whole-cell Ca²⁺ current density between the two treatments (FIG. 2D). The data confirm that ApoCIII solely impinge on β cell Ca_(V)1 channels.

Apolipoprotein CIII Hyperactivates β Cell Ca_(V) Channels via Coactivation of PKA and Src Kinase

The increase in open probability of β cell Ca_(V)1 channels by ApoCIII and the mediating role of protein kinases in ApoCIII signaling suggest that ApoCIII may signal upstream of some protein kinases to hyperactivate β cell Ca_(V) channels (Gui et al., 2006; Kawakami et al., 2006; Rueckschloss and Isenberg, 2004; Waitkus-Edwards et al., 2002; Wu et al., 2001). Therefore, we explored the involvement of PKA, PKC and Scr kinase in ApoCIII-induced hyperactivation of β cell Ca_(V) channels.

First, we examined the effect of the PKA inhibitor H-89 on ApoCIII-induced hyperactivation of β cell Ca_(V) channels in RINm5F cells. Whole-cell Ca²⁺ currents registered in control cells were larger than those in cells treated with ApoCIII, whereas whole-cell Ca²⁺ currents recorded in cells incubated with ApoCIII plus H-89 sized in between (FIG. 3A). Average Ca²⁺ current densities measured in ApoCIII-treated cells (filled circles, n=36) were significantly higher than those in vehicle control cells (open circles, n=37) at voltages ranging from 10 to 50 mV (FIG. 3B). However, cells following cotreatment of ApoCIII and H-89 (filled triangles, n=36) did not significantly differ from either cells treated with ApoCIII or control cells in terms of Ca²⁺ current density (FIG. 3B). Moreover, H-89 treatment did not significantly influence Ca²⁺ current densities under basal conditions, i.e. in the absence of ApoCIII (FIGS. 7A and B). The results indicate that PKA inhibition marginally reduced ApoCIII-induced hyperactivation of β cell Ca_(V) channels.

Second, we tested the effect of the PKC inhibitor calphostin C (CalpC) on ApoCIII-induced hyperactivation of β cell Ca_(V) channels in RINm5F cells. We observed that cells incubated with ApoCIII and ApoCIII/CalpC-cotreated cells displayed similar whole-cell Ca²⁺ currents, which were larger than those acquired in vehicle-treated cells (FIG. 3C). Mean Ca²⁺ current densities in ApoCIII-treated cells (filled circles, n=33) at the voltage range 10-50 mV and cells exposed to ApoCIII/CalpC (filled triangles, n=33) at a voltage range from 20 to 50 mV increased significantly in comparison with vehicle control cells (open circle's, n=33) (FIG. 3D). There is no difference between ApoCIII-treated cells and ApoCIII/CalpC-cotreated cells with regard to the Ca²⁺ current density (FIG. 3D). Furthermore, cells exposed to control vehicle were similar to a CalpC-treated cells in terms of Ca²⁺ current density (FIGS. 7C and D). The data demonstrate that PKC inhibition does not affect ApoCIII-induced hyperactivation of β cell Ca_(V) channels.

Third, we evaluated the effect of the Src kinase inhibitor PP2 on ApoCIII-induced hyperactivation of β cell Ca_(V) channels in RINm5F cells. We found smaller and larger whole-cell Ca²⁺ currents in cells following incubation with vehicle solution and ApoCIII-incubated cells, respectively (FIG. 3E). Cells exposed to Appall and PP2 fell between vehicle control cells and cells treated with ApoCIII with regard to whole-cell Ca²⁺ currents (FIG. 3E). Whole-cell Ca²⁺ current densities quantified in cells treated with ApoCIII (filled circles, n=40) at the voltage range 10-50 mV were significantly elevated as compared with those determined in vehicle control cells (open circles, n=40) (FIG. 3F). Cells subjected to cotreatment of ApoCIII and PP2 (filled triangles, n=40) showed significantly larger Ca²⁺ currents at the voltage range 20-40 mV than vehicle control cells (open circles, n=40). However, the difference in the Ca²⁺ current density between ApoCIII/PP2-cotreated cells and cells incubated with vehicle solution is less prominent than that between cells treated with ApoCIII and vehicle control cells (FIG. 3F). Moreover, vehicle-treated cells (open circles, n=20) and cells incubated with PP2 (filled circles, n=19) exhited similar Ca²⁺ current densities (FIGS. 7E and F). The results suggest that Src kinase inhibition has a tendency to decrease ApoCIII-induced hyperactivation of β cell Ca_(V) channels.

The marginal and null effects of PKA, PKC or Src kinase inhibitors on ApoCIII-induced hyperactivation of β cell Ca_(V) channels made us wonder what happens if a more complex inhibition of all these kinases is applied. To address this question, we characterized the effect of the protein kinase inhibitor cocktail H-89, CalpC and PP2 on ApoCIII-induced hyperactivation of β cell Ca_(V) channels in RINm5F cells. Larger whole-cell Ca²⁺ currents appeared in an ApoCIII-treated cells, whereas smaller whole-cell Ca²⁺ currents occurred in vehicle control cells and cells treated with ApoCIII in the presence of H-89, CalpC and PP2 (FIG. 4A). ApoCIII treatment (filled circles, n=35) significantly increased Ca²⁺ current densities at the voltage range 10-50 mV as compared with vehicle control (open circles, n=35) and treatment with ApoCIII together with H-89, CalpC and PP2 (filled triangles, n=34). The profile of Ca²⁺ current densities in cells exposed to ApoCIII in the presence of H-89, CalpC and PP2 resembled that in vehicle control cells (FIG. 4B). Furthermore, treatment of control cells with the protein kinase inhibitor cocktail H-89, CalpC and PP2 had no significant effect on whole-cell Ca²⁺ currents under basal conditions, i.e. in the absence of ApoCIII (FIGS. 8A and B). The results demonstrate that combined inhibition of PKA, PKC and Src kinase effectively ablates ApoCIII-induced hyperactivation of β cell Ca_(V) channels.

The marginal effect of PKA or Src kinase inhibitors alone on whole-cell Ca²⁺ currents inevitably raised the question if coinhibition of PKA and Src kinase is sufficient to prevent ApoCIII-induced hyperactivation of β cell Ca_(V) channels. We answered the question by analyzing whole-cell Ca²⁺ currents in RINm5F cells following cotreatment of II-89 and PP2. We observed that whole-cell Ca²⁺ currents in ApoCIII-heated cells were larger than those in control cells or cells subjected to treatment of ApoCIII in the presence of II-89 and PP2 (FIG. 4C). Significantly higher densities of whole-cell Ca²⁺ currents appeared in the ApoCIII group (filled circles, n=26) in comparison with control group (open circles, n=26) or group subjected to incubation with ApoCIII in the presence of H-89 and PP2 (filled triangles, n=27) (FIG. 4D). Moreover, whole-cell Ca²⁺ currents in control cells resembled those observed in cells treated with H-89 and PP2 (FIGS. 5C and D). The data reveal that ApoCIII enhances whole-cell Ca²⁺ currents via coactivation of PKA and Src Kinase.

Apolipoprotein CIII does not Influence β Cell Ca_(V)1 Channel Expression

Overnight incubation with ApoCIII may influence β cell Ca_(V)1 channel expression. To test for this possibility, we analyzed β cell Ca_(V)1 channel expression in RINm5F cells following ApoCIII incubation. We found that anti-Ca_(V)1.2, anti-Ca_(V)1.3 and anti-GAPDH antibodies detected clear Ca_(V)1.2, Ca_(V)1.3 and GAPDH immunoreactive bands, respectively. Control and ApoCIII-treated samples gave similar intensities of Ca_(V)1.2, Ca_(V)1.3 and GAPDH immunoreactivities (FIG. 5A). FIG. 5B shows that there was no significant difference in the relative abundance of Ca_(V)1.2 (hatched column, n=6) and Ca_(V)1.3 subunits (filled column, n=6) in RINm5F cell homogenates subjected to ApoCIII incubation in comparison with vehicle incubation (open column, n=6) (P>0.05). The data reveal that ApoCIII incubation did not alter β cell Ca_(V)1 channel expression at the protein level.

Apolipoprotein CIII Upregulates β Cell Ca_(V) Channels via β1 Integrin

β1 integrin has been verified to serve as a mediator between ApoCIII and a certain number of protein kinases including PKA and Src kinase (Gui et al., 2006; Kawakami et al., 2006; Rueckschloss and Isenberg, 2004; Waitkus-Edwards et al., 2002; Wu et al., 2001). This together with our results that ApoCIII hyperactivated β cell Ca_(V) channels via coactivation of PKA and Src kinase raises the possibility for β1 integrin to mediate ApoCIII-induced hyperactivation of β cell Ca_(V) channels. We investigated this possibility by implementing RNA interference in combination with whole-cell Ca⁺ analysis in RINm5F cells. It turned out that transfection with two β1 integrin siRNAs significantly decreased β1 integrin expression at the protein level (FIGS. 6A and B). We also visualized that whole-cell Ca²⁺ currents detected in cells subjected to negative control siRNA transfection and ApoCIII exposure (NC siRNA/apoCIII) were larger than those in other cells following different treatments (FIG. 6C). These treatments included mock transfection and control vehicle incubation (NC siRNA/Control), negative control siRNA transfection and exposure to control vehicle (NC siRNA/Control), β1 integrin siRNA transfection and treatment with vehicle solution (β1 integrin siRNA/Control), and β1 integrin siRNA transfection and ApoCIII incubation (β1 integrin siRNA/ApoCIII). Cells subsequent to NC siRNA/apoCIII (filled triangles, n=28) showed a significant increase in Ca²⁺ current density at the voltage range 10-40 mV in comparison with NO siRNA/Control (n=29), NC siRNA/Control (n=28), β1 integrin siRNA/Control (n=29), and β1 integrin siRNA/ApoCIII (n=29) (FIG. 6D). The difference in Ca²⁺ current density between NC siRNA/apoCIII and β1 integrin siRNA/apoCIII was less than that between NC siRNA/apoCIII and other treatments (FIG. 6D). Taken together, the results demonstrate that ApoCIII critically relies on β1 integrin to hyperactivate β cell Ca_(V) channels.

Discussion

The gross conductivity of Ca_(V) channels depends on the density and activity of functional channels in the plasma membrane of the cell. Enhancement of whole-cell Ca²⁺ currents by type 1 diabetic serum and its factor ApoCIII can result from enriched density and/or increased conductivity of functional Ca_(V) channels in the β cell plasma membrane (Juntti-Berggren et al., 1993; Juntti-Berggren et al., 2004). However, all studies (Juntti-Berggren et al., 2004; Ristic et al., 1998; Yang and Berggren, 2005; Yang and Berggren, 2006) except one (Juntti-Berggren et al., 1993) have so far examined the effect of type 1 diabetic serum on Ca_(V) channels only at the whole cell level. In the study by Juntti-Berggren et al, the increase in β cell Ca_(V) channel activity by type 1 diabetic serum was characterized at both the single channel and the whole-cell level (Juntti-Berggren et al., 1993). Unfortunately, this work did not analyze whether type 1 diabetic senun could alter the density of functional Ca_(V) channels in the β cell plasma membrane (Juntti-Berggren et al., 1993). Although we have previously revealed that ApoCIII serves as a type 1 diabetic serum factor, hyperactivating β cell Ca_(V) channels, only whole-cell patch-clamp analysis was performed (Juntti-Berggren et al., 2004). Undoubtedly, detailed examination of biophysical properties of single Ca_(V) channels in ApoCIII-treated cells should be implemented to mechanistically dissect hyperactivation of β cell Ca_(V) channels by this apolipoprotein. Interestingly, cell-attached single channel recordings in the present work reveals that incubation with ApoCIII not only augments the activity of individual β cell Ca_(V)1 channels but also enriches the number of functional Ca_(V)1 channels in the recorded area of the β cell plasma membrane. The augmentation of single Ca_(V)1 channel activity is visualized as an increased open probability attributed to the prolonged mean open time and shortened mean closed time. Enrichment of number of functional Ca_(V)1 channels is verified by appearance of more levels of single Ca_(V)1 channel conductance.

The insulin-secreting RINm5F cell is equipped with Ca_(V)1, Ca_(V)2 and Ca_(V)3 channels (Yang and Berggren, 2005; Yang and Berggren, 2006). We investigated if ApoCIII selectively hyperactivates Ca_(V)1 channels or indiscriminately impacts all these three types of Ca_(V) channels in this insulin-secreting cell. It turned out that ApoCIII-induced hyperactivation of β cell Ca_(V) channels could no longer take place following pharmacological ablation of Ca_(V)1 channels. This means that ApoCIII selectively hyperactivates Ca_(V)1 channels, which are the major Ca_(V) channel type playing a predominant role over other types of Ca_(V) channels in β cell physiology and pathophysiology. The selective hyperactivation of β cell Ca_(V)1 channels by ApoCIII accounts for the pathophysiological role of this lipoprotein in Ca²⁺-dependent β cell death (Juntti-Berggren et al., 2004; Yang and Berggren, 2005; Yang and Berggren, 2006).

A series of protein kinases, such as PKA and PKC, can effectively phosphorylate Ca_(V) channels resulting in increases in the open channel density and activity due to phosphorylation-induced conformational changes in these channels (Catterall, 2000; Kavalali et al., 1997; Yang and Tsien, 1993). Increases in the number and open probability of functional Ca_(V) channels by ApoCIII might be mediated by protein kinases. ApoCIII has been demonstrated to activate PKC through β1 integrin in monocytic cells (Kawakami et al., 2006). Furthermore, β1 integrin activation can also upregulate Ca_(V)1 channels in neurons, ventricular myocytes and vascular smooth muscle cells through stimulation of PKA, PKC and Src kinase (Gui et al., 2006; Rueckschloss and Isenberg, 2004; Waitkus-Edwards et al., 2002; Wu et al., 2001). All these components are present in β cells (Bosco et al., 2000; Kantengwa et al., 1997; Mukai et al., 2011; Nikolova et al., 2006; Yang and Berggren, 2006) and may suggest that. ApoCIII employs the β1 integrin-PKA/PKC/Src kinase cascade to hyperactivate β cell Ca_(V) channels. Indeed, the present work shows that complex inhibition of PKA, PKC and Src kinase effectively abrogates ApoCIII-induced hyperactivation of β cell Ca_(V) channels and that coinhibition of PKA and Src kinase is enough for this effect. However, individual inhibition of PKA, PKC or Src kinase only produced, if anything, a marginal effect on ApoCIII-induced hyperactivation of β cell Ca_(V) channels. Hence, we conclude that ApoCIII relies on parallel PKA and Src pathways to upregulate β cell Ca_(V) channels.

Occurrence of ApoCIII-induced hyperactivation of β cell Ca_(V) channels requires overnight incubation. Hence, the effect might be accounted for by an increase in Ca_(V) channel expression. Therefore, we quantified immunoreactivities of Ca_(V)1.2 and Ca_(V)1.3 subunits in RINm5F cells following overnight incubation with ApoCIII. However, the incubation had no influence on β cell Ca_(V)1 channel expression. We therefore excluded the possibility that ApoCIII elevates β cell Ca_(V)1 channel expression.

The transmembrane receptor β1 integrin is noncovalently associated with other integrins to form a set of heterodimers. They recognize a large number of soluble and surface-bound proteins to mediate cell-cell, cell-extracellular matrix and cell-pathogen interactions (Luo et al., 2007). β1 Integrin is situated downstream of ApoCIII and upstream of PKA/PKC/Src kinase in some cell types (Gui et al., 2006; Kawakami et al., 2006; Rueckschioss and Isenberg, 2004; Waitkus-Edwards et al., 2002; Wu et al., 2001). This made us investigate whether the ApoCIII-β1 integrin-PKA/PKC/Src kinase pathway operates in the β cell as the mechanism whereby this apolipoprotein hyperactivates Ca_(V)1 channels. Interestingly, knockdown of β1 integrin does not influence β cell Ca_(V) channel activity in the absence of ApoCIII, but significantly abrogates ApoCIII-induced hyperactivation of β cell Ca_(V) channels. The results clearly verify that β1 integrin plays a significant role in mediating the action of ApoCIII on β cell Ca_(V)1 channel activity.

In conclusion, our findings demonstrate that ApoCIII selectively hyperactivates β cell Ca_(V)1 channels through parallel PKA and Src kinase pathways in a β1 integrin-dependent fashion. ApoCIII-induced hyperactivation of β cell Ca_(V)1 channels is characterized by the enriched density and increased activity of functional Ca_(V)1 channels in the β cell plasma membrane. Undoubtedly, this novel signal-transduction pathway has a potential to serve as an innovative drug discovery platform for the prevention of Ca²⁺-dependent β cell death in association with diabetes.

Experimental Procedures

Cell Culture and Treatments

Islets of Langerhans were isolated from adult male and female mice and dispersed into single β cells. RINm5F cells at about 70% confluency were trypsinized. The resultant suspension of cells was seeded into Petri dishes or 12-well plates. The cells were cultivated in RPMI 1640 medium supplemented with 10% fetal bovine serum, 2 mM L-glutamine, and 100 U/100 μg/ml penicillin/streptomycin (Invitrogen, Carlsbad, Calif.) and maintained at 37° C. in a humidified 5% CO₂ incubator. They were grown overnight and then subjected to siRNA transfection. For patch-clamp analysis, cells underwent overnight treatment with ApoCIII, the PKA inhibitor H-89 (Calbiochem, La Jolla, Calif.), the PKC inhibitor calphostin C (Calbiochem), the Src kinase inhibitor PP2 (Calbiochem) and the Ca_(V)1 channel blocker nimodipine (Calbiochem) in RPMI medium at final concentrations of 20 μg/ml, 0.5 μM, 0.1 μM, 0.1 μM, and 5 μM, respectively. ApoCIII was dissolved in 0.1% trifluoroacetic acid (TFA) to make a stock solution of 1 mg/ml, whereas H-89, calphostin C, PP2 and nimodipine were dissolved in dimethyl sulfoxide (DMSO) to form stock solutions of 5 mM, 1 mM, 1 mM and 10 mM, respectively. 0.002% TFA and/or 0.03% DMSO were used as vehicle controls.

siRNA Design and Transfection

Two pairs of 21-mer siRNA duplexes targeting the rat β1 integrin (β1 integrin siRNA #1, ID127971 and β1 integrin siRNA #2, ID127972) were designed and chemically synthesized by Applied Biosystems/Ambion (Austin, Tex.) Their sequences were subjected to BLAST search to ensure their specificity. Silencer® Select Negative Control siRNA (4390843), not targeting any gene product, and Silencer® Select GAPDH Positive Control siRNA (4390849), efficiently silencing GAPDH in human, mouse, and rat cells, were purchased from Applied Biosystems/Ambion (Austin, Tex.). RINm5F cells were reversely transfected with Lipofectamine™ RNAiMAX™. Briefly, negative control siRNA, β1 integrin siRNA #1 or β1 integrin siRNA #2 was mixed with Lipofectamine™ RNAiMAX™ followed by 20-min incubation at room temperature. Subsequently, cells were added to the siRNA/Lipofectamine™ RNAiMAX mixtures followed by gentle agitation and kept at 37° C. in a humidified 5% CO₂ incubator. After 72 h, the transfected cells were grown to about 70% confluency and subjected to immunoblot assay or different treatments

SDS-PAGE and Immunoblot Analysis

RINm5F cells following different treatments were lysed in a lysis buffer (pH 7.5) consisting of 50 mM HEPES, 150 mM NaCl, 1 EGTA, 1 mM EDTA, 10% glycerol, 1% triton X-100, 1 mM PMSF and a protease inhibitor cocktail (Roche Diagnostics, Mannheim, Germany). The lysate was centrifuged at 800×g for 10 min at 4° C. to remove cell debris and nuclei. The protein concentration of the resulting samples was determined with Bio-Rad protein assay reagent (Bio-Rad, Hercules, Calif.). The samples were then denatured by heating at 96° C. for 3 min in SDS sample buffer and underwent sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analysis. Briefly, 50, 90 or 180 μg proteins were separated in discontinuous gels consisting of a 4% acrylamide stacking gel (pH 6.8) and an 8% acrylamide separating gel (pH 8.8). The separated proteins were then electroblotted to hydrophobic polyvinylidene difluoride membrane (Hybond-P; GE Healthcare, Uppsala, Sweden). The blots were blocked by incubation for 1 h with 5% non-fat milk powder in a washing buffer, containing 50 mM tris(hydroxymethyl)aminomethane, 150 mM NaCl and 0.05% Tween 20 (pH 7.5). They were then incubated overnight at 4° C. with affinity-purified rabbit polyclonal antibodies to β1 integrin (1:500; Millipore, Billerica, Mass.), Ca_(V)1.2 (1:200) and Ca_(V)1.3 (1:200), respectively, and for 1 h at room temperature with mouse monoclonal antibody to glyceraldehyde-3-phosphate dehydrogenase (GAPDH, 1:4000; Applied Biosystems/Ambion, Austin, Tex.), respectively. After rinsing with the washing buffer, the blots were incubated with the secondary antibodies (either horseradish peroxidase-conjugated goat anti-rabbit IgG or horseradish peroxidase-conjugated goat anti-mouse IgG; 1:50,000; Bio-Rad, Hercules, Calif.) at room temperature for 45 min. The immunoreactive bands were visualized with the ECL plus Western blotting detection system (GE Healthcare, Uppsala, Sweden).

Electrophysiology

Mouse islet cells and RINm5F cells following different treatments were subjected to single channel and whole-cell patch-clamp measurements. Cell-attached and perforated whole-cell patch-clamp configurations were employed. Electrodes were made from borosilicate glass capillaries, fire-polished and coated with Sylgard close to their tips. Some of them were filled with a solution containing (in mM) 110 BaCL₂, 10 TEA-Cl, and 5 HEPES (pH 7.4 with Ba(OH)₂) for single channel measurements. Others were filled with a solution composed of (in mM) 76 Cs₂SO₄, 1 MgCl₂, 10 KCl, 10 NaCl, and 5 HEPES (pH 735 with CsOH), as well as amphotericin B (0.24 mg/ml) for whole-cell current recordings. Electrode resistance ranged between 4 and 6 MΩ when they were filled with electrode solutions and immersed in bath Solutions. The electrode offset potential was corrected in bath solutions prior to gigaseal formation. Single-channel recordings were performed with cells bathed in a depolarizing external recording solution, containing (in mM) 125 KCl, 30 KOH, 10 EGTA, 2 CaCl₂, 1 MgCl₂, and 5 HEPES-KOH (pH 7.15). This solution was used to bring the intracellular potential to 0 mV. For perforated whole-cell current measurements, the cells were bathed in a solution containing (in mM) 138 NaCl, 5.6 KCl, 1.2 MgCl₂, 10 CaCl₂, 5 HEPES (pH 7.4). Single channel and whole-cell currents were recorded with an Axopatch 200B amplifier (Molecular Devices, Foster City, Calif.) and an EPC-9 patch clamp amplifier (HEKA Elektmnik, Lambrecht/Pfalz, Germany), respectively, at room temperature (about 22° C.). Acquisition and analysis of single channel and whole-cell current data were done using the software program pCLAMP 10 (Axon Instruments) and the software program PatchMaster/FitMaster (HEKA), respectively. The amplitude of whole-cell currents was normalized by the cell capacitance.

Statistical Analysis

All data are presented as mean±SEM. Statistical significance was determined by one-way ANOVA, followed by least significant difference (LSD) test. When two groups were compared, unpaired Student's t test or Mann-Whitney U test was employed. The significance level was set at 0.05 or 0.01.

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We claim:
 1. An in vitro method for identifying candidate compounds for limiting development of and/or treating diabetes, comprising (a) contacting a population of insulin secreting cells with an amount of apolipoprotein CIII (ApoCIII) effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of one or more test compounds; and (b) identifying those positive test compounds that inhibit an ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the population of insulin secreting cells compared to control, wherein the control comprises contacting a control population of insulin secreting cells with an amount of ApoCIII effective to increase density and/or conductivity of Ca_(v)1 channels, in the presence of the one or more test compounds and further contacting the control population of insulin secreting cells with a Src kinase inhibitor and a protein kinase A (PKA) inhibitor, wherein positive test compounds that inhibit the ApoCIII-induced increase in density and/or conductivity of Ca_(v)1 channels in the population of insulin secreting cells to a greater degree than in the control population of insulin secreting cells are candidate compounds for limiting development of and/or treating diabetes.
 2. The method of claim 1 wherein the insulin-secreting cells are pancreatic β cells.
 3. The method of claim 1, wherein the method comprises contacting the cells with ApoCIII for at least 6 hours.
 4. The method of claim 1, wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 1 diabetes.
 5. The method of claim 1, wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 2 diabetes.
 6. The method of claim 2, wherein the method comprises contacting the cells with ApoCIII for at least 6 hours.
 7. The method of claim 2, wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 1 diabetes.
 8. The method of claim 2, wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 2 diabetes.
 9. The method of claim 6, wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 1 diabetes.
 10. The method of claim 6, wherein the candidate compounds are candidate compounds for limiting development of and/or treating type 2 diabetes. 